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United States Patent |
6,264,866
|
Yamada
,   et al.
|
July 24, 2001
|
Method for producing polyimide film
Abstract
A method for producing a polyimide film in which the imidation ratio and/or
the amount of volatile constituent are controlled to improve the adhesive
strength of the polyimide film. The method may also comprise controlling
the highest temperature of heating the prefilm to improve the adhesive
strength of the polyimide film.
Inventors:
|
Yamada; Hirofumi (Otsu, JP);
Fukudome; Manabu (Otsu, JP);
Egawa; Naoki (Otsu, JP);
Kondo; Yuzuru (Ashiya, JP);
Maki; Haruhiko (Kyoto, JP)
|
Assignee:
|
Kanegafuchi Kagaku Kogyo Kabushiki Kaisha (Osaka, JP)
|
Appl. No.:
|
095129 |
Filed:
|
June 10, 1998 |
Foreign Application Priority Data
Current U.S. Class: |
264/216; 264/204; 264/236; 264/331.19; 264/347 |
Intern'l Class: |
B29C 039/14; B29C 041/24 |
Field of Search: |
264/216,204,236,331.21,347,331.19
|
References Cited
U.S. Patent Documents
4470944 | Sep., 1984 | Asakura | 264/216.
|
4797307 | Jan., 1989 | Kunimoto et al. | 264/216.
|
5324475 | Jun., 1994 | Okahashi et al. | 264/216.
|
Primary Examiner: Vargot; Mathieu D.
Attorney, Agent or Firm: Pillsbury Winthrop LLP
Claims
What is claimed is:
1. A method for producing an adhesive polyimide film comprising:
casting a composition into a film shape, wherein said composition consists
substantially of an organic solvent solution of polyamide acid and
chemical curing agents selected from the group consisting of dehydrating
agents and tertiary amines;
heating the film shaped composition at an initial temperature of
200.degree. C. or less, and thereafter increasing the temperature in a
step-wise fashion such that solvent is evaporated to form a chemically
cured prefilm while adjusting an imidation ratio represented by the
formula:
##EQU3##
and
further heating said prefilm to obtain an adhesive polyimide film.
2. A method for producing an adhesive polyimide film comprising:
casting a composition into a film shape, wherein said composition consists
substantially of an organic solvent solution of polyamide acid and one or
more chemical curing agents selected from the group consisting of a
dehydrating agent and a tertiary amine;
heating the film shaped composition at an initial temperature of
200.degree. C. or less, and thereafter increasing the temperature in a
step-wise fashion such that solvent is evaporatiod to form a chemically
cured prefilm while adjusting amounts of volatile constituent; and
further heating said prefilm to obtain an adhesive polyimide film.
3. A method for producing an adhesive polyimide film comprising:
casting a composition into a film shape, wherein said composition consists
substantially of an organic solvent solution of polyamide acid and
chemical curing agents selected from the group consisting of dehydrating
agents and tertiary amines;
heating the film shaped composition at an initial temperature of
200.degree. C. or less, and thereafter increasing the temperature in a
step-wise fashion such that solvent is evaporatiod to form a chemically
cured prefilm while adjusting amounts of organic solvent and an imidation
ratio represented by the formula:
##EQU4##
and
further heating said prefilm to obtain an adhesive polyimide film.
4. A method for producing a polyimide film according to claim 1 or claim 3,
wherein said adjusted imidation ratio of the prefilm is 70% or more.
5. A method for producing a polyimide film according to claim 2 or claim 3,
wherein said adjusted amount of volatile constituent of the prefilm is 40
weight % or less.
6. A method for producing a polyimide film according to claim 3, wherein
said adjusted imidation ratio of the prefilm is 70% or more and said
adjusted amount of volatile constituent of the prefilm is 40 weight % or
less.
7. A method for producing a polyimide film according to any of claims 1, 2
or 3, wherein the highest temperature of heating a prefilm is in a range
from 450.degree. C. to 630.degree. C.
8. A method for controlling adhesiveness of a polyimide film comprising:
casting a composition into a film shape, wherein said composition consists
substantially of an organic solvent solution of polyamide acid and
chemical curing agents selected from the group consisting of dehydrating
agents and tertiary amines;
heating the film shaped composition at an initial temperature of
200.degree. C. or less, and thereafter increasing the temperature in a
step-wise fashion such that solvent is evaporatiod to form a chemically
cured prefilm while adjusting an imidation ratio represented by the
formula:
##EQU5##
and
further heating said prefilm to obtain an adhesive polyimide film.
9. A method for controlling adhesiveness of a polyimide film comprising:
casting a composition into a film shape, wherein said composition consists
substantially of an organic solvent solution of polyamide acid and one or
more chemical curing agents selected from the group consisting of a
dehydrating agent and a tertiary amine;
heating the film shaped composition at an initial temperature of
200.degree. C. or less, and thereafter increasing the temperature in a
step-wise fashion such that solvent is evaporatiod to form a chemically
cured prefilm while adjusting amounts of volatile constituent; and
further heating said prefilm to obtain an adhesive polyimide film.
10. A method for controlling adhesiveness of a polyimide film comprising:
casting a composition into a film shape, wherein said composition consists
substantially of an organic solvent solution of polyamide acid and
chemical curing agents selected from the group consisting of dehydrating
agents and tertiary amines;
heating the film shaped composition at an initial temperature of
200.degree. C. or less, and thereafter increasing the temperature in a
step-wise fashion such that solvent is evaporatiod to form a chemically
cured prefilm while adjusting amounts of organic solvent and an imidation
ratio represented by the formula:
##EQU6##
and
further heating said prefilm to obtain an adhesive polyimide film.
11. The method according to claim 8 or claim 10, wherein the adjusted
imidation ratio of the prefilm is 70% or more.
12. The method according to claim 9 or claim 10, wherein the adjusted
amount of volatile constituent of the prefilm is 40 weight % or less.
13. The method according to claim 10, wherein the adjusted imidation ratio
of the prefilm is 70% or more and the adjusted amount of volatile
constituent of the prefilm is 40 weight % or less.
14. The method according to any one of claims 8-10, wherein the highest
temperature for heating a prefilm is in a range from 450.degree. C. to
630.degree. C.
Description
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to a method for producing a polyimide film,
more particularly, to a method for producing a polyimide film by means of
controlling its adhesive property.
2. Disclosure of the Related Art
As is commonly known, a polyimide film has various excellent properties
such as heat-resistance, cold resistance, chemical-resistance, insulation
and mechanical strength. Therefore, a polyimide film is widely used as
such materials as an electric insulating film, a heat insulating film and
a base film of flexible printed wiring board. In order to use a polyimide
film for various purposes, a copper-clad laminated board is produced by
superposing a layer of adhesive agent and copper foil on one or a both
sides of a polyimide film, a pre-impregnated film (prepreg) is produced by
coating a polyimide film with adhesive agent, or a composite film is
produced by laminating fluorocarbon resin on a polyimide film. The
adhesive strength is one of the important properties of a film, so that
various attempts to improve the adhesive strength of a film have been
made.
In order to provide the adhesive strength to a surface of a polymeric film,
there are various techniques such as flame treatment for a film surface,
corona discharge treatment, ultraviolet treatment, alkaline treatment,
primer treatment, sand blast treatment and plasma treatment. For the
purpose of providing the adhesive strength to a polyimide film without
spoiling its other properties, one or more of the above-mentioned
treatments are selected. These treatments are considered to be used for
removing a weak boundary layer, which is formed on the film surface in the
course of solvent-cast process and lowers the adhesive strength of a film.
All of the above-mentioned treatments are executed after producing a film
to improve the adhesive strength of a finished film. The adhesive strength
of a finished film is not constant. The above-mentioned after-treatments
are also unstable. Therefore, it is difficult to stably provide a film
with an improved adhesive strength by a prior art method. There is also an
inevitable problem that the cost goes up due to the additional step to
improve an adhesive property of a finished film. Same troubles and
problems also occur in the course of producing a composite of a polyimide
film and fluorocarbon resin. Thus, it has been difficult to constantly
provide high adhesive strength to a film by using the conventional
methods.
OBJECTS AND SUMMARY OF THE INVENTION
As the result of our researches to remove the above disadvantages and to
stably provide a film with a high adhesive strength, we have eventually
found the method for producing a film having an excellent adhesive
property in which an adhesive property is controlled in course of
producing a film. In the conventional method, an adhesive property of a
film is improved by way of treating the surface of a finished film,
however, in the present invention, an excellent adhesive property is
incorporated in course of producing a film.
The method for producing a polyimide film according to the present
invention, which overcomes the above-discussed and numerous other
disadvantages and deficiencies of the prior art, comprising the steps of
i) casting a composition into a film shape, wherein said composition
consists substantially of organic solvent solution of polyamide acid; ii)
heating the film-shaped composition to obtain a prefilm with an adjusted
imidation ratio and/or an adjusted amount of volatile constituent; and
iii) further heating said prefilm to obtain a polyimide film.
The adjusted imidation ratio of the prefilm can be 70% or more.
The adjusted amount of volatile constituent of the prefilm can be 40 weight
% or less.
The adjusted imidation ratio of the prefilm can be 70% or more and the
adjusted amount of volatile constituent of the prefilm can be 40 weight %
or less.
The highest temperature of heating a prefilm can be in a range from
450.degree. C. to 630.degree. C.
Thus, the present invention disclosed herein makes possible the objectives
of (1) providing an adhesive property directly to a polyimide film; (2)
providing an adhesive property extremely stably to a polyimide film for
the purpose of overcoming uneven quality of adhesiveness and sudden loss
of adhesiveness, which are often the case with conventional methods for
providing adhesiveness to a finished film; and (3) maintaining a high
value of the adhesive strength of a composite film obtained by laminating
fluorocarbon resin on a polyimide film.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a flowchart of the present method for producing a polyimide film
with an excellent adhesive property.
FIG. 2 is an example of the belt chamber according to the present
invention.
FIG. 3 is a graph showing the relation between the amount of volatile
constituent of a prefilm and the adhesive strength of a finished product.
(The imidation ratio is not fixed.)
FIG. 4 is a graph showing the relation between the imidation ratio of a
prefilm and the adhesive strength of a finished product. (The amount of
volatile constituent is not fixed.)
DESCRIPTION OF THE PREFERRED EMBODIMENTS
A "prefilm" used herein means a gel-state film hardened to such an extent
that it can support itself after allowing a composition including the
solvent solution of precursor of polyimide to flow from a die and then
placing it on a supporter consecutively (in other words, casting the
composition over the supporter); and evaporating a solvent or a reaction
product to promote the imidation. The above-mentioned composition
including the solvent solution of precursor of polyimide consists
substantially of organic solvent solution of polyamide acid.
The preferred embodiments of the present method for producing a film with
an excellent adhesive property are described below.
Generally, a polyimide film is insoluble and infusible, so that a solvent
casting method, in which organic solvent solution of precursor of
polyimide is cast over a supporter such as a drum and a belt, is used for
producing a polyimide film. The solvent casting method is also used in the
present invention.
A polyimide film produced by using the present method can be obtained from
well-known materials, e.g., from one or more organic tetracarboxylic
dianhydrides and one or more diamines.
In the present invention, the term "a polyimide film" is interpreted in a
broad sense, including polyimide, polyamideimide, polyetherimide and
polyesterimide films. The term includes polyimide films of
non-thermoplasticity, thermoplasticity and thermosetting. In other words,
molecular structure of polyimide is not restricted in the present
invention. However, it is preferable that a polyimide consists of a
repeating unit of the general formula (1):
##STR1##
wherein R.sub.1 is a tetravalent organic group. Concretely, R.sub.1 has at
least one benzene, which bonds directly to an adjoining carbonyl group.
More concretely, R.sub.1 is at least one selected from the group
consisting of
##STR2##
wherein X is a divalent functional group selected from the group consisting
of
##STR3##
and R.sub.4 is CH.sub.3 --, Cl--, Br--, F--, or CH.sub.3 O-- and R.sub.4
can be the same or the different in the case that more than two of them
are substituted.
R.sub.2 is a divalent organic group, which has at least one benzene. More
concretely, R.sub.2 is at least one selected from group consisting of
##STR4##
##STR5##
wherein R.sub.4 is CH.sub.3 --, Cl--, Br--, F--, or CH.sub.3 O-- and
R.sub.4 can be the same or different kind in the case that more than two
of them are substituted.
One of the preferred polyimides contains R.sub.1 represented by
##STR6##
and R.sub.2 represented by
##STR7##
as the main component respectively. A polyimide copolymer containing
R.sub.1 represented by
##STR8##
and R.sub.2 represented by
##STR9##
and
##STR10##
as the main component respectively is also preferred.
Moreover, it is preferable that the molar ratio of organic group
represented by
##STR11##
to organic group represented by
##STR12##
is raging from 50 to 50 to 90 to 10.
##STR13##
It is also preferable that a polyimide copolymer contains R.sub.1
represented by
##STR14##
and R.sub.2 represented by
##STR15##
or R.sub.1 represented by
##STR16##
and
##STR17##
and R.sub.2 represented by
##STR18##
and
##STR19##
as the main component respectively.
Polyimide resin represented by the above general formula (1) can be
obtained by the dehydrating closure reaction of a polyamide acid polymer,
which is a precursor of polyimide resin. Polyamide acid solution may be
obtained by polymerizing an approximately equal mole of dianhydride and
diamine in organic polar solvents according to the conventional method.
One example of the methods for producing polyamide acid is described below.
Under an inert atmosphere consisting of, e.g., argon gas, nitrogen gas or
the like, at least one kind of acid dianhydrides selected from aromatic
tetracarboxylic dianhydrides represented by the general formula (2);
##STR20##
wherein R.sub.1 is a tetravalent organic group, is dissolved or diffused in
organic solvents to make a solution. At least one kind of diamine
ingredients represented by the general formula (3), wherein R.sub.2 is a
bivalent organic group is added to the solution to obtain a polyamide acid
polymer. The diamine ingredients can be dissolved or diffused in an
organic solvent prior to adding the solution. Alternatively, the diamine
in a solid state may be added to the solution. It is preferable that the
reacting temperature ranges from -10.degree. C. to 50.degree. C. and the
reacting time ranges from 30 minutes to 6 hours.
Alternatively, first, one or more diamines may be dissolved or diffused in
an organic solvent to make a solution. Then one or more acetic
dianhydrides are added to the solution to obtain a polyimide acid
solution. Also, diamines and acetic dianhydrides can be mixed in an
organic solution simultaneously.
It is preferable that the average molecular weight of polyimide resin is
more than 10,000 in order to maintain the strength of polyimide resin. In
many cases, it is difficult to directly measure molecular weight of a
polyimide polymer. In such cases, molecular weight is estimated
indirectly. For example, in the case that a polyimide polymer is
synthesized by polyamide acid, molecular weight of polyimide is considered
to be equivalent to that of polyamide acid.
As aromatic tetracarboxylic dianhydride represented by the general formula
(2),
##STR21##
various types of aromatic tetracarboxylic dianhydride can be used. More
concretely, in due consideration of balance of various properties, one or
more aromatic tetracarboxylic dianhydrides can be selected, wherein
R.sub.1 of the general formula (2) is a tetravalent organic group selected
from the group consisting of
##STR22##
Wherein X is a bivalent functional group represented by
##STR23##
R.sub.4 is CH.sub.3 --, Cl--, Br--, F--, or CH.sub.3 O-- and R.sub.4 can be
the same or different kind in the case that two or more of them are
substituted.
Various types of diamines can be used as a diamine compound represented by
the general formula (3).
H.sub.2 N--R.sub.2 --NH.sub.2 (3)
More concretely, in due consideration of balance of various properties, one
or more diamines can be selected, wherein R.sub.2 of the general formula
(3) is a bivalent organic group selected from the group consisting of
##STR24##
##STR25##
wherein R.sub.4 is CH.sub.3 --, Cl--, Br--, F--, or CH.sub.3 O-- and
R.sub.4 can be the same or different kind in the case that two or more of
them are substituted. The examples of organic solvents, which can be used
in producing polyamide acid, are as follows: sulfoxide solvents such as
dimethylsulfoxide and diethylsulfoxide; formamide solvents such as
N,N-dimethylformamide and N,N-diethylformamide; and acetamide solvents
such as N,N-dimethylacetamide and N,N-diethylacetamide. One of the
above-mentioned organic solvents can be used alone or the combination of
two or more of them can be used as mixed organic solvents. Also, the
combination of the above polar solvents and inactive solvents of polyamide
acid can be used as mixed organic solvents. Examples of such inactive
solvents are acetone, methanol, ethanol, isopropanol, benzene,
2-methoxyethanol toluene, xylene, THF and the like.
Polyimide can be produced from polyamide acid obtained by means of the
above-mentioned reaction. Polyimide can be obtained by a chemical-curing
method, in which polyamide acid, a precursor of polyimide, is imidated by
being heat-treated under the presence of both dehydrating agent
represented by acid anhydride including acetic anhydride and tertiary
amine including picoline, quinoline, isoquinoline and pyridine.
Alternatively, polyimide can be also obtained by means of the heat-curing
method, in which imidating reaction proceeds without dehydrating agent and
tertiary amine. Since imidating reaction takes place more rapidly in the
chemical-curing method, the chemical-curing method can be more productive
and profitable than the heat-curing method. Moreover, polyimide obtained
from the chemical-curing method has advantages of high mechanical strength
and small coefficient of linear expansion.
The use of the chemical-curing method followed by the heat-treating
treating method makes possible speedier imidation. Therefore, it is
preferable to produce polyimide film by the combined use of the
chemical-curing method and the heat-curing method. In the present
invention, said combined use is introduced to the process of imidation.
As shown in FIG. 1, the chemical-curing method is used in the first half of
the process and the heat-curing method in the latter half of the process,
in which imidation completes. Therefore, "a prefilm" of the present
invention is defined as a film obtained after the chemical-curing process
but before the heat-curing process.
Next, one example of methods for producing a polyimide film according to
the present invention is concretely described below. Typically, the
methods for producing a polyimide film by imidating a precursor of
polyimide may consist of two processes. The one may be the chemical-curing
process executed in the drum or the belt chamber or the equivalent
thereof, and the other may be the heat-curing process executed in the
tenter chamber or the equivalent thereof.
In the process executed in the belt chamber, a precursor of polyimide mixed
by the blender is allowed to flow from the T-die. The precursor of
polyimide allowed to flow from the T-die is placed onto the endless belt
or on the casting drum to be formed into the shape of film while being
hardened. The film-shaped precursor is moved and heated in accordance with
the rotation of the belt or the drum while imidation of the film is
promoted. In the belt chamber, reaction products which are combustible
volatile constituents consisting mainly of acetic acid and organic
solvents are transpired.
In the belt chamber, the temperature of atmosphere and rotation speed of
the belt or the drum are adjusted for the purpose of prevention of
ignition to the combustible volatile constituent transpired from the
polyimide resin or to the resin itself. A warm breeze, hot blast, radiant
heat, belt heat and the like can be used in the belt chamber.
In the course of the above process, imidation of a film-shaped composition
is promoted. After being heated and dried to such an extent that it can
support itself, the film-shaped composition is exfoliated from the endless
belt to obtain a prefilm.
The inventors of the present invention found out that the amount of
volatile constituent and the imidation ratio have a remarkable effect
directly on the improvement of an adhesive property of the fished film.
Finally, we succeeded in stably providing a film with an excellent
adhesive property at a reasonable price by controlling the amount of
volatile constituent and the imidation rate of a prefilm within a certain
range.
In the case of successive processes, a film-shaped composition is treated
in the tenter chamber by using the heat-curing method after being
exfoliated from the drum or the belt. This film-shaped composition
exfoliated from the drum or the belt can be defined as a prefilm. However,
the embodiment of the present invention is by no means limited to the
above and various embodiments are applicable. Depending on the situation,
an additional heating apparatus including the heat roll can be also
applied to the embodiment in order to promote chemical-curing of a film.
In other words, a prefilm can be defined as a film obtained before being
put in the tenter chamber or the equivalent thereof in which heat-curing
is completed.
In the present invention, the amount of volatile constituent of a prefilm
is calculated by using the following formula;
##EQU1##
W: the weight of a prefilm before being dried
W.sub.0 : the weight of a prefilm after being heat-treated at 450.degree.
C. for 20 minutes
It is preferable that the amount of volatile constituent of a prefilm
exfoliated from a supporter such as the endless belt and the drum should
be adjusted to a range of 20-200 weight %. When the amount of volatile
constituent of a prefilm is less than 40 weight %, the adhesive strength
of a finished polyimide film is improved.
The imidation rate of a prefilm is calculated on the basis of absorbance of
a sample prefilm measured by using the method of infrared absorption
spectrum. To prepare a sample, a film of about 50 mm in length and about
100 mm in width is cut out of a prefilm and then soaked in methanol
solution with 5 weight % of aniline for about an hour. The solution is
then changed into methanol and the film is soaked in methanol for about
half an hour for three times. Then the film is washed and dried at a room
temperature. The absorbance of the sample prepared as such is measured by
using the device for Fourier transform infrared absorption spectrum
(System 2000, available from Perkin Elmer Co. Ltd.) under the condition of
ATM prism KRS-5, a prism entrance angle of 45.degree., measuring range of
4000 cm.sup.-1.about.4500 cm.sup.-1, ten times of measuring and resolution
of 4.00. The imidation rate of the sample is calculated by using the
following formula;
##EQU2##
n: absorbance of a sample at 1374 cm.sup.-1 /absorbance of a sample at 1498
cm.sup.-1
n.sub.x : n value of a measured film
n.sub.100 : n value of completely (100%) imidated film
Absorbance at 1374 cm.sup.-1 is characteristic of an imido group and
absorbance at 1498 cm.sup.-1 is characteristic of benzene ring. A
100%-imidized film is obtained by forming a composition consisting of a
polymer and organic solvent solution identical to the sample into a film
shape and then heating the film-shaped composition at 110.degree. C. for
120 seconds, 300.degree. C. for 30 seconds, 450.degree. C. for 70 seconds,
500.degree. C. for 30 seconds and then 500.degree. C. for 5 minutes.
Preferably, the imidation ratio of a prefilm exfoliated from the heated
supporter such as the endless belt and the drum is adjusted to 70% or
more. An adhesive property of a finished polyimide film is improved by
adjusting the imidation ratio in a range of 70% or more. The films made of
a prefilm of 70% or more imidation ratio have much higher adhesive
strength than that made of prefilm of less than 70% imidation ratio.
The amount of volatile constituent and the imidation ratio can be adjusted
by controlling the temperature and the heating time in the belt chamber.
Normally, in the chemical-curing process, heating is executed to promote a
reaction. However, in order to exclusively promote chemical-curing rather
than heat-curing the temperature is maintained 200.degree. C. or less. It
is preferable to gradually raise the temperature of the atmosphere in the
belt chamber step by step, so that solvent and reaction product are
evaporated. A sudden rise of the temperature causes wrinkles on the film
surface due to the difference of the drying speed between the surface and
the inside of the film. Also, it causes undesired exfoliation due to
partial hardening of the edge.
The temperature in the belt chamber is adjusted depending on the kind and
the thickness of a film and the kind of a solvent to be used. The
temperature is gradually increased to nearly a boiling point of a solvent.
For example, in the case that dimethylformamide is used as a solvent, as a
boiling point of dimethylformamide is 153.degree. C., the temperature in
the belt chamber is adjusted to 50.degree. C. to 150.degree. C. The
highest temperature in the belt chamber has a potent influence on an
adhesive property of a finished film. Higher maximum temperature produces
a higher adhesive property of a finished film.
Heating time also depends on the thickness of a film, the kind of a
film-shaped composition and the highest temperature in the belt chamber.
Higher maximum temperature can reduce heating time and production cost.
Next, the belt chamber is concretely described below. The preferable belt
chamber is separated into several rooms to differentiate the temperatures
between the rooms. In the process shown in FIG. 2, the belt chamber
consists of: a parallel stream solidifying room 10, jet stream solidifying
rooms 12, an exfoliation room 14. In the room 10, a film cast over the
supporter is heated up and then exposed to a gas stream parallel to a film
to be solidified to such an extent that the film loses its fluidity. The
room 12 consists of one or more sections, in which the film treated in the
room 10 is exposed to gas blowing against it to be solidified to such an
extent that it can support itself. In the room 14, the film treated in the
room 12 is exfoliated from the endless belt 20.
More specifically, a film-shaped composition of polyimide precursor solvent
solution from the die 22 is conveyed to the endless belt 20 which may be
made from stainless-steel, aluminum-alloy or the like. The pulleys 18 over
which the belt 20 is trained contain cooling devices. Then, gas 24 is sent
parallel to the surface of the film-shaped composition heated up evenly.
The film-shaped composition is gradually solidified by gas 24 to such an
extent that a film loses its fluidity in the room 10, while volatile
constituent and organic solvents produced in this process are being
transpired. The so obtained film-shaped composition is then conveyed to
the rooms 12, in which the film-shaped composition is heated by one or
several stages, while the jets of gas is blown against the film-shaped
composition from nozzles resulting in transpiring volatile constituent and
organic solvents. The so treated film-shaped composition, or a prefilm,
which is solidified and dried to such an extent that it can support itself
is exfoliated from the endless belt 20 in the room 14. This prefilm is
heat-treated in the tenter chamber in the next step.
The amount of volatile constituent of the prefilm before heat-treating in
the tenter chamber is preferably 40 weight % or less, and more preferably
30 weight % or less in order to improve an adhesive property of a finished
film.
Moreover, in the present invention, the imidation ratio of a prefilm is
preferably 70% or more, and more preferably 80% or more in order to
improve an adhesive property of a finished film.
An adhesive property of a finished polyimide film can be improved by
controlling the amount of volatile constituent and the imidation ratio of
a prefilm as mentioned above. The amount of volatile constituent and the
adhesive strength are correlated, and so are imidation ratio and the
adhesive strength. It is presumed that volatile constituent which hinders
the adhesive strength of a finished film can be effectively removed by
holding down the amount of volatile constituent of a prefilm.
In the next process, the prefilm exfoliated from the endless belt is
heat-treated in the tenter chamber with its edges fixed. The tenter
chamber can consist of a heating furnace and a device for lowering the
temperature gradually. However, it is by no means limited to the above. In
the tenter chamber, the film can be moved by shifting a sheet fixed by
pins in accordance with rotating drive of a pin conveyer. Imidation of a
prefilm is promoted by gradual heat-treatment of the prefilm in the
heating furnace, in which heat-curing is executed. Normally,
heat-treatment is carried out at the starting temperature of about
200.degree. C. and then the temperature is gradually raised so as to
complete imidation of a prefilm. Thus, a finished polyimide film is
obtained.
The highest temperature of the heat-treatment is preferably in a range from
450.degree. C. or more to 630.degree. C. or less, and more preferably from
520.degree. C. or more to 580.degree. C. or less. A temperature gradient
to the highest temperature can be any form without any special
restriction. In the case that the highest temperature is less than
450.degree. C., the adhesive strength of a finished film is not
effectively improved. In the case that the highest temperature is more
than 630.degree. C., mechanical properties including the adhesive strength
is extremely spoiled, which is considered to be caused by heat
deterioration of a polyimide film.
As shown in the above embodiment, a prefilm exfoliated from the endless
belt can be heat-treated at the highest temperature of 450.degree. C. or
more to 630.degree. C. or less successively as the final stage of the
whole process. Alternatively, a finished polyimide film produced by the
use of the conventional methods can be heat-treated at the temperature of
450.degree. C. or more to 630.degree. C. However, in order to simplify the
process and to avoid high production cost, the above heat-treatment is
preferred to be carried out in course of producing polyimide.
The heat-treatment can be carried out for few seconds to few minutes,
preferably for ten seconds to ten minutes, and more preferably for ten
seconds to three minutes, which depends on the heating temperature. For
example, heat-treating a polyimide film at 600.degree. C. for ten minutes
causes heat deterioration. For another example, heat-treating a polyimide
at 500.degree. C. for five seconds does not produce sufficient results on
the improvement of an adhesive property.
A polyimide film, which is completely imidized in the above heat-curing
process, gradually cooled down in the device for lowering the temperature.
As described above, the method for heat-treating a film at a very high
temperature ranging from 450.degree. C. or more to 630.degree. C. or less
has never been proposed as a method for improving an adhesive property of
a polyimide film in the prior arts. Therefore, the present invention is a
novel invention to improve an adhesive property of a finished polyimide
film by quite simple heat-treatment. The mechanism is not completely
cleared up at the moment, however, an adhesive property is supposed to be
improved with the result that a dense layer is formed on the surface of
the film by heat-treating at the high temperature.
A polyimide film obtained by using the method of the present invention
includes a sheet-like film with a various thickness ranging from a few
micrometers to hundreds of micrometers. The thickness of a film can be
selected in accordance with the purpose of use. For example, a film with
the thickness of 12.5 .mu.m to 50 .mu.m is used as a base film of a
flexible printed wiring board.
The present invention is a method to improve an adhesive property of a
finished film by controlling the imidation ratio and the amount of
volatile constituent of a prefilm. Moreover, the present invention makes
possible to stably and economically provide a film with a higher adhesive
property by heat-treating at the high temperature in the tenter chamber.
The present method for producing a polyimide film is described above as an
example of the embodiments of the method. However, this invention is by no
means limited thereto. As the need arises, other well-known
after-treatment such as flame treatment, corona discharge treatment,
ultraviolet treatment, alkaline treatment, primer treatment, sandblast
treatment and plasma treatment can be combined with the method of the
present invention.
EXAMPLES
The present invention will be more clearly understood by referring to the
Examples below. However, the Examples should not be construed to limit the
invention in any way. In the Examples, corona discharge treatments were
executed against polyimide films at 220.+-.10 W.multidot.min/m.sup.2 of
electric power density with the use of aluminum electrode. The adhesive
strength of the polyimide film was evaluated by executing the following
measuring method: using acrylic adhesive agent "Pyralux" (a product and
trade name of E. I. du Pont de Nemours & Co., Inc.), the polyimide film
was laminated with a copper foil having 35 .mu.m of thickness "3EC" (an
electrolyzed copper foil, a product of Mitui Metal & Mining Co., Ltd.),
and then the above adhesive agent was allowed to react at 185.degree. C.
for an hour to be hardened to produce FCCL (flexible copper-clad
laminate); a test sample was cut out of the FCCL so that width of copper
pattern of the FCCL could become 3 mm, and then the sample was subject to
a tension test via 90.degree. of exfoliation at 50 mm/min. of peeling
speed by applying a tension tester "S-100-C", a product of Shimazu
Seisakusho, Co., Ltd. The results of the average of five measurements are
shown in Table 1.
Examples 1.about.12
The imidation rate and the amount of volatile constituent of a prefilm, and
the adhesive strength of a finished product were measured. The heating
temperature and time at the stage of chemical curing varied while
conditions at the stage of heat-curing were constant.
Polyamide acid solution was prepared from 4,4'-diaminodiphenylether
selected from aromatic diamines and pyromellitic acid dianhydride selected
from aromatic tetracarboxylic dianhydrides. Acetic anhydride and
isoquinoline were mixed quickly with the solution. The molar rate of the
acetic anhydride to a repeating unit of the polyamide acid was 5.5 to 1,
and the molar ratio of the isoquinoline to a repeating unit of the
polyamide acid was 0.55 to 1. Then the mixture was formed into a
film-shaped composition on the aluminum foil with the thickness of 30
.mu.m. Each film-shaped composition was heated for 30 to 360 seconds at
the temperature ranging from 90 to 160.degree. C. in an oven under the 12
different conditions, and prefilms with the thickness of 25 .mu.m were
finally obtained. Then the prefilms were heat-treated at the temperature
of 300.degree. C. for 30 seconds and then at the temperature of
500.degree. C. for a minute. Thus the imidation of the prefilms was
completed and the finished films were subjected to corona discharge
treatment to obtain PI film products.
The amount of volatile constituent and the imidation ratio concerning each
prefilm as well as the adhesive strength of each corresponding P1 film
product were measured. The results are shown in Table 1.
TABLE 1
volatile adhesive
temperature time constituent imidation strength
(.degree. C.) (sec.) (%) ratio (%) (kg/cm)
Example 1 90 90 216 78 0.79
Example 2 90 180 37 80 1.34
Example 3 90 360 28 80 1.36
Example 4 110 60 256 66 0.79
Example 5 110 90 61 85 1.22
Example 6 110 120 31 87 1.40
Example 7 110 180 25 88 1.43
Example 8 110 240 22 88 1.51
Example 9 160 30 202 80 0.99
Example 10 160 60 25 85 1.54
Example 11 160 90 17 91 1.63
Example 12 160 120 15 94 1.70
Based on the results, the relation between the amount of volatile
constituent of prefilms and the adhesive strength of the P1 film products
as well as the relation between the imidation ratio of prefilms and the
adhesive strength of the P1 film products are shown in FIGS. 3 and 4,
respectively.
Table 1 shows that higher temperature in the belt chamber or longer heating
time in the case of same temperature makes the amount of volatile
constituent decreased and the imidation ratio increased. Table 1, FIG. 3
and FIG. 4 show that less amount of volatile constituent or higher
imidation ratio of a prefilm makes the adhesive strength of the finished
product increased.
Examples 13.about.15
Polyamide acid solution was obtained from 4,4'-diaminodiphenylether and
para-phenylenediamine selected from aromatic diamines in the molar rate of
3:1 and pyromellitic acid dianhydride selected from aromatic
tetracarboxylic dianhydrides. Acetic anhydride and isoquinoline were added
to the solution. The molar rate of the acetic anhydride to a repeating
unit of the polyamide acid was 5.0 to 1, and the molar rate of the
isoquinoline to a repeating unit of the polyamide acid was 0.51 to 1. Then
the mixture was stirred sufficiently and adjusted to 0.degree. C. to make
dope.
The so obtained dope was allowed to flow from a T-die and formed into a
film-shaped composition with the thickness of about 250 .mu.m on the
smooth metal endless belt consecutively. Then it was dried up by hot air
while the belt was rotated. The amount of volatile constituent of three
prefilms, each of which was prepared under the conditions that the
temperature of the belt chamber was set at 85.degree. C./95.degree.
C./100.degree. C., 95.degree. C./105.degree. C./110.degree. C. and
110.degree. C./120.degree. C./125.degree. C. (the total drying time is
about 100 seconds), respectively, were measured.
The prefilms exfoliated from the belt were heat-treated at the temperature
ranging from 250.degree. C. to 550.degree. C. for about 80 seconds in
total in the tenter chamber. Then the films were gradually cooled down to
room temperature in the cooling room. After that, corona discharge
treatment was executed against each film and the adhesive strength of the
25 .mu.m-thick polyimide film obtained under the above three temperature
conditions was measured. The results of the measurement are shown in Table
2.
TABLE 2
volatile adhesive
constituent strength
(%) (kg/cm)
Example 13 140 1.2
Example 14 90 1.3
Example 15 40 1.4
Table 2 shows that as the amount of volatile constituent of a prefilm
decreases, the adhesive strength of a corresponding film increases.
Examples 16.about.18
Next, polyimide films were produced under the various conditions of maximum
temperature in the heat-curing treatment process in a tenter chamber,
while the condition of chemical curing remained constant.
Polyamide acid solution was prepared from 4,4'-diaminodiphenylether
selected from aromatic diamines and pyromellitic acid dianhydride selected
from aromatic tetracarboxylic dianhydrides. Acetic anhydride and
isoquinoline were mixed quickly with the solution. The molar ratio of the
acetic anhydride to a repeating unit of the polyamide acid was 5.5 to 1,
and the molar ratio of the isoquinoline to a repeating unit of the
polyamide acid was 0.55 to 1. Then the mixture was formed into a
film-shaped composition. After that, the film-shaped composition was
heat-treated at the temperature of 120.degree. C. for 100 seconds to
obtain a prefilm. Then the prefilm was treated at 270.degree. C. for 30
seconds and at 370.degree. C. for 30 seconds (the temperature was
gradually raised). The resulting film was then baked for 30 seconds at the
maximum temperature of 500.degree. C., 550.degree. C. and 630.degree. C.,
respectively. Consequently, three kinds of 25 .mu.m-thick polyimide film
were produced under three different conditions.
Examples 19.about.21
Polyamide acid solution was obtained from 4,4'-diaminodiphenylether and
para-phenylenediamine selected from aromatic diamines in the molar ratio
of 3:1 and pyromellitic acid dianhydride selected from aromatic
tetracarboxylic dianhydrides. Acetic anhydride and isoquinoline were mixed
quickly with the solution. The molar ratio of the acetic anhydride to a
repeating unit of the polyamide acid was 5.0 to 1, and the molar ratio of
the isoquinoline to a repeating unit of the polyamide acid was 0.51 to 1.
Then the mixture was formed into a film-shaped composition. After that,
the film-shaped composition was heat-treated at the temperature of
120.degree. C. for 100 seconds to obtain a prefilm. Then it was
heat-treated at 270.degree. C. for 30 seconds and at 370.degree. C. for 30
seconds (the temperature was gradually raised). The resulting film was
baked for 30 seconds at the maximum temperature of 500.degree. C.,
570.degree. C. and 630.degree. C., respectively. Consequently, three kinds
of 25 .mu.m-thick polyimide films were produced under three different
conditions.
The adhesive strength of each 25 m-thick polyimide film obtained in Example
16 to 21 was evaluated. The results are shown in Table 3.
TABLE 3
maximum adhesive
temperature time strength
(.degree. C.) (sec.) (kg/cm)
Example 16 500 30 0.6
Example 17 550 30 0.7
Example 18 630 30 0.7
Example 19 500 30 0.6
Example 20 570 30 0.8
Example 21 630 30 0.8
Table 3 shows that the higher maximum temperature produces higher adhesive
strength.
A polyimide films which were heat-treated at the various temperatures under
the equal maximum condition of 450.degree. C. were compared as follows.
Example 22
Polyamide acid solution was prepared from 4,4'-diaminodiphenylether
selected from aromatic diamines and pyromellitic acid dianhydride selected
from aromatic tetracarboxylic dianhydrides. Acetic anhydride and
isoquinoline were mixed quickly with the solution. The molar ratio of the
acetic anhydride to a repeating unit of the polyamide acid was 5.5 to 1,
and the molar ratio of the isoquinoline to a repeating unit of the
polyamide acid was 0.55 to 1. Then the mixture was formed into a
film-shaped composition. After that, the film-shaped composition was
heat-treated at the temperature of 120.degree. C. for 100 seconds to
obtain a prefilm. Then it was heat-treated at 270.degree. C. for 30
seconds and at 370.degree. C. for 30 seconds (the temperature was
gradually raised). The resulting film-shaped composition was then baked
for a minute at the maximum temperature of 450.degree. C. Consequently, 25
.mu.m-thick polyimide film was produced.
Example 23
Polyamide acid solution was obtained from 4,4'-diaminodiphenylether and
para-phenylenediamine selected from aromatic diamines in the molar ratio
of 3:1 and pyromellitic acid dianhydride selected from aromatic
tetracarboxylic dianhydrides. Acetic anhydride and isoquinoline were mixed
quickly with the solution. The molar ratio of the acetic anhydride to a
repeating unit of the polyamide acid was 5.0 to 1, and the molar ratio of
the isoquinoline to a repeating unit of the polyamide acid was 0.51 to 1.
Then the mixture was formed into a film-shaped composition. After that,
the film-shaped composition was heat-treated at the temperature of
120.degree. C. for 100 seconds to obtain a prefilm. Then it was
heat-treated at 270.degree. C. for 30 seconds and at 370.degree. C. for 30
seconds (the temperature was gradually raised). The resulting film-shaped
composition was then baked for a minute at the maximum temperature of
450.degree. C. Consequently, a 25 .mu.m-thick polyimide film was produced.
The adhesive strength of a 25 .mu.m-thick polyimide film obtained in
Example 22 to 23 was measured. The results are shown in Table 4.
TABLE 4
maximum adhesive
temperature time strength
(.degree. C.) (min.) (kg/cm)
Example 22 450 5 0.3
Example 23 450 5 0.3
These results show that the adhesive strength of the film heat-treated at a
low maximum temperature is lower than that of the film heat-treated at a
high maximum temperature.
Examples 24.about.25
Polyimide films obtained in the Examples 22 and 23 were heat-treated at the
temperature of 550.degree. C. and 570.degree. C. which were equal
condition of the Examples 17 and 20, and then the adhesive strength of
each film was measured. The results are shown in Table 5.
TABLE 5
adhesive
strength
additional treatment (kg/cm)
Example 24 Example 22 + 550.degree. C. .times. 30 sec. 0.7
Example 25 Example 23 + 570.degree. C. .times. 30 sec. 0.8
These results show that the adhesive strength of a polyimide film obtained
in the Example 22 or 23 was provided with as high adhesive strength as
that of a film obtained in the Example 17 or 20.
Examples 26.about.28
Polyamide acid solution was obtained from 4,4'-diaminodiphenylether and
para-phenylenediamine selected from aromatic diamines in the molar ratio
of 3:1 and pyromellitic acid dianhydride selected from aromatic
tetracarboxylic dianhydrides. Acetic anhydride and isoquinoline were added
to the solution. The molar ratio of the acetic anhydride to a repeating
unit of the polyamide acid was 5.8 to 1, and the molar ratio of the
isoquinoline to a repeating unit of the polyamide acid was 0.59 to 1. Then
the mixture was stirred sufficiently and adjusted to 0.degree. C. to make
dope.
The so obtained dope was allowed to flow from a T-die and formed into a
film-shaped composition with the thickness of about 250 .mu.m on the
smooth metal endless belt consecutively. Then it was dried up by hot air
while the belt was rotated. The temperature conditions of the belt chamber
was set at 110.degree. C./120.degree. C./125.degree. C. and the total
drying time was about 100 seconds.
The prefilms exfoliated from the endless belt were heat-treated in the
tenter chamber at the temperature starting from 250.degree. C. up to
500.degree. C., 530.degree. C., or 560.degree. C. (gradually heated up)
for about 80 seconds in total. Then the films were gradually cooled down
to room temperature in the cooling room. After that, corona discharge
treatment was executed against each film and the adhesive strength of the
25 .mu.m-thick polyimide film obtained under the above three temperature
conditions was measured. The results of the measurement are shown in Table
6.
TABLE 6
maximum adhesive
temperature strength
(.degree. C.) (kg/cm)
Example 26 500 1.3
Example 27 530 1.4
Example 28 560 1.5
These results show that higher maximum temperature produces higher adhesive
strength.
In this way, inventors of the present invention pay their attention to the
relation between the amount of volatile constituent of a prefilm or the
imidation ratio and the adhesive property of a finished film, and the
relation between temperature condition of heat-treating of the prefilm and
the adhesive property, to which no attention has been given so far.
This invention makes it possible to enhance the adhesive strength of a
finished polyimide film by controlling the amount of volatile constituent
and the imidation ratio of a prefilm; the temperature and time of
heat-treating; and the maximum temperature in a heat-curing process.
Having now fully described the invention, it will be apparent to one of
ordinary skills in the art that various changing and modifications can be
made thereto without departing from the spirit or scope of the present
invention as set forth herein.
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